Investigation of Atomic Oxygen Interactions with Amorphous Silica Surfaces under VLEO Conditions

Satellite surfaces in very low Earth orbit (VLEO) experience drag and surface erosion from hyperthermal (5 eV ≈ 7.8 km/s) collisions of reactive oxygen atoms abundant at these altitudes. In the free-molecular VLEO regime, gas-surface interactions (GSI) between atomic oxygen and satellite materials predominantly govern the orbital aerodynamics. The dynamics of scattering depend on several factors, including incident angle and energy of the gas particle, surface temperature, and chemical composition. Accurately capturing this behavior is crucial for predicting drag and informing strategies to reduce or compensate for the aerodynamic drag. The present work performs molecular dynamics (MD) simulations of atomic oxygen (AO) interacting with an amorphous silica surface (SiO2) at an incident energy representative of VLEO conditions. The reactive force field (ReaxFF) potential is employed to generate an amorphous, pristine silica surface through thermal annealing. To represent long-term AO exposure on satellite surfaces, steady-state adsorption of an AO flux is applied to produce an AO-saturated silica surface. These two surfaces serve as targets for trajectory simulations of incident atomic oxygen. The study compares the scattering dynamics of pristine and AO-saturated surfaces, investigating AO interactions through sequential adsorption and scattering events. Scattering simulations are performed at two limiting incident angles, 5° and 90°, with 5000 independent AO impacts per angle, each randomized over a plane 10 Å above the silica surface. Initial results indicate that tangential incidence yields predominantly elastic and specular reflection, whereas normal incidence results in significant adsorption and diffusive scattering of reflected atoms. The angular and energy distributions reveal key surface interaction mechanisms influencing momentum and energy transfer. The study provides physically grounded data to support the development of DSMC-compatible GSI models for accurate prediction of satellite drag in rarefied VLEO conditions.